A parking brake system is installed in a vehicle for restricting a vehicle movement during its parking condition. There is a manually operated parking brake system, according to which an operational force is transmitted to a brake device by pulling a brake cable by means of a brake lever. There is an electrically operated parking brake system, according to which a rotational force of an electric motor is transmitted to the brake device by pulling the brake cable by use of the electric motor.
According to an electronic parking brake (EPB) system, which is one of the electrically operated parking brake systems, an electric motor is rotated in a direction for a locked condition (in a forward direction) in order to transmit the rotational force of the electric motor to a brake device when locking parking brake thereof, and the rotation of the electric motor is stopped while generated braking force is maintained. When un-locking (releasing) the parking brake, the electric motor is rotated in a direction for un-locked (released) condition (in a reversed direction), so that the parking brake is released.
According to a conventional electronic parking brake system, such as disclosed in Japanese Patent Publication No. 2002-205627, an electric motor is rotated in a direction of a locked condition until electric current to the electric motor reaches at a predetermined value when locking the parking brake. On the other hand, the electric motor is rotated in a direction of an un-locked (released) condition until a number of rotation of the electric motor reaches at a predetermined value when un-locking (releasing) the parking brake. In other words, the number of rotation of the electric motor for releasing the parking brake (the rotation in the reversed direction) corresponds to a number of rotation of the electric motor for locking the parking brake. Namely, the number of rotation of the electric motor in case of releasing the parking brake corresponds to a cable length, for which the brake cable is pulled when locking the parking brake.
According to another conventional electronic parking brake system, such as disclosed in Japanese Patent Publication No. 2005-329930, an electric motor is likewise rotated in a direction of a locked condition until electric current to the electric motor reaches at a predetermined value when locking the parking brake. A stroke of a brake device is measured during the operation for locking the parking brake, and then the electric motor is rotated in a releasing direction until the rotation of the electric motor reaches at a value, which is obtained by adding an amount corresponding to an elongation of a brake to the above measured stroke.
In the above first mentioned conventional electronic parking brake system (No. 2002-205627), a releasing (un-locking) operation of the parking brake is ended when the number of motor rotation reaches at such an amount corresponding to the length of the brake cable, for which the brake cable was pulled by the electric motor during the locking operation. However, in such a control, a control error may become larger, as a result of being influenced by elongation of the brake cable.
In the above second mentioned conventional electronic parking brake system (No. 2005-329930), the stroke of the brake device is measured during the locking operation, and the electric motor is rotated in the reversed direction (the releasing direction), until the rotation of the electric motor in the reversed direction corresponds to the value, which is obtained by adding the amount corresponding to the elongation of the brake cable to the above measured stroke. However, in such a control, although the elongation of the brake cable is taken into consideration, the releasing operation may not be sufficiently carried out when the parking brake is maintained in the locked condition for a long period. In other words, when the elongation of the brake cable may become larger than that estimated during such a longer period, such elongation may affect the precise control for the releasing operation. Then, the control error may happen to occur.
When the control error may occur, a partial braking condition may be continued as a result of insufficient releasing operation. Namely, it is a problem that a braking force is still generated even when the locked condition of the parking brake (the locked condition of the EPB system) has been released.